The present invention relates to electrographic printing apparatus, and more particularly to such apparatus for the printing of images by first forming an electrostatic latent image on an imaging member, such as a belt or drum, and then toning the latent image to develop a toned image which may be transferred to a recording sheet to form a finished print. In particular it relates to such systems wherein the electrostatic latent image is formed by depositing a pattern of charge on the imaging member.
In one class of prior art constructions, it is known to deposit a pointwise pattern of charge by actuating selective pins of an electrostatic pin array. Other mechanisms of charge deposition now commonly used include the deposition of a pointwise charge pattern by "ionographic" print cartridges. These systems are printers having printheads wherein a matrix of sets of electrodes are provided, each set constituting an "ion generator" which forms and accelerates charged particles--ions, electrons or both--toward the imaging member. Descriptions of systems of this type may be found in U.S. Pat. Nos. 4,155,093; 4,628,227 and many others. Such constructions create harsh environmental conditions as a result of the glow discharges which are the source of the charge carriers, and these conditions adversely affect the lifetime and performance of key components.
In the past, it has been proposed to utilize electrons for printing by employing some conventional electron-generating structure, such as a cathode ray tube, to form the desired latent image; more recently there have been a number of proposals to employ microlithographically-made electron sources for charge pattern formation. As representative of this class of electron sources, reference is made to U.S. Pat. Nos. 4,259,678; 4,858,062; 4,810,934; and 4,904,895. Such sources might, for example, be used to write a pattern on a cathode ray tube (CRT) or on a liquid crystal display (LCD). Typically, in proposed constructions of this type, the electron source is operated in a vacuum, and to be of use the electrons must generally be accelerated to an energy in the tens of kilovolts to actuate the phosphors of a CRT screen or to pass through an electron-transmissive face plate or window. The provision of a vacuum region or of electron-transmissive windows in an electrographic imaging apparatus, however, may raise problems of cost, complexity, reliability or even safety.
Perhaps for these reasons, applicant is unaware of commercial print systems based on electron writing sources, except for such electron effects as occur in operating the aforesaid ionographic printing systems at high voltages to deposit negative charge carriers.